The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Methods for efficiently compressing an image signal include JPEG, and JPEG-2000 for the still image, and H.261, H.263, MPEG-2, and MPEG-4 for the dynamic image. Further, MPEG-4 AVC (Advanced Video Coding) for providing the more improved compression efficiency was standardized by the ISO (International Standardization Organization) in 2003 while succeeding technologies such as MPEG-2, and MPEG-4, etc.
Data encoding for the video image includes intra prediction encoding and inter prediction encoding. The intra prediction encoding or the inter prediction encoding is an effective method in order to reduce the correlation between data and it is widely used for compressing various types of data. Especially, because a motion vector of a current block determined by estimating movements of a current block to be currently encoded in the inter prediction encoding has a high correlation with motion vectors of adjacent blocks, it is possible to first calculate a predicted value (hereinafter, referred to as a Predicted Motion Vector: PMV) for a motion vector of the current block by using the motion vectors of the adjacent blocks and then encode not the true value of the motion vector of the current block but only a differential value (hereinafter, referred to as a Differential Motion Vector: DMV) with regard to the predicted value. Therefore, it is possible to considerably reduce the amount of bits to be encoded, which improves the encoding efficiency.
That is, according to most of the known image compression standards such as MPEG-4 AVC, in inter prediction encoding, an encoder encodes only the differential motion vector, which is the differential value between the current motion vector and the predicted motion vector determined by estimating movements of the current block in a reference frame reconstructed through previous encoding and decoding, and then transmits the encoded DMV. A decoder predicts the motion vector of the current block by using the motion vectors of the adjacent blocks decoded in advance, and reconstructs the current motion vector by adding the transmitted differential motion vector to the predicted motion vector.
However, the inventor(s) has noted that when the motion vector is encoded in accordance with the known image compression standard described above, if there is no correlation between the predicted motion vector and the current motion vector, the differential motion vector is increased. Therefore, the inventor(s) has experienced that the generated bit amount is increased, and the compression efficiency is decreased. The inventor(s) has also experienced that when the encoder additionally encodes information of the predicted motion vector in order to reduce the differential motion vector, a bit amount required for encoding additional information is increased, so that the compression efficiency is decreased.